Browsing by Author "Silva, A."
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- Acoustic maritime rapid environmental assessment 2004 during the MREA'04 sea trialPublication . Jesus, S. M.; Soares, C.; Felisberto, P.; Silva, A.; Farinha, L.; Martins, C.Environmental inversion of acoustic signals for bottom and water column properties is being proposed in the literature as an interesting concept for complementing direct hydrographic and oceanographic measurements for Rapid Environmental Assessment (REA). The acoustic contribution to REA can be cast as the result of the inversion of ocean acoustic properties to be assimilated into ocean circulation models specifically tailored and calibrated to the scale of the area under observation. Traditional ocean tomography systems and methods for their requirements of long and well populated receiving arrays and precise knowledge of the source/receiver geometries are not well adapted to operational Acoustic REA (AREA). The Acoustic Oceanographic Buoy (AOB) was proposed as an innovative concept that responds to the operational requirements of AREA. That concept includes the development of water column and geo-acoustic inversion methods being able to retrieve environmental true properties from signals received on a drifting network of acoustic-oceanographic sensors - the AOBs. An AOB prototype and a preliminary version of the inversion code, was tested at sea during the Maritime Rapid Environmental Assessment 2003 (MREA’03) sea trial and was reported in [1]. On a separate register it should be noted that the characterization of the environment between the source and the receiver also contributes to the identification of the acoustic channel response and therefore provides a basis for fulfilling the objectives of project NUACE1. The present report describes the data sets and results gathered during the MREA’04 sea trial that took place from 29 March to 19 April 2004 off the west coast of Portugal, south of Lisboa (Portugal), with the objectives of testing an improved version of the individual AOB and its functionality in a simple network. The acoustic part of the experiment lasted for four days between April 7 and April 10, 2004 and involved the transmission and reception of pre-coded signals along range-dependent and range-independent acoustic tracks.
- Acoustic oceanographic buoy (version 1)Publication . Silva, A.; Martins, C.; Jesus, S. M.One of the known impairements in the application of ocean acoustic tomography in operational scenarios has been the size, weight and difficulty of operation of actual ocean going equipment, such as hydrophone arrays and acoustic sources. The Acoustic Oceanographic Buoy - AOB is meant to be, at its final stage, an easy to deploy and easy to maintain autonomous vertical array that puts together in a single system acoustic and non-acoustic sensors, self-storing of geotime and GPS referred data, on the buoy preprocessing capabilities and network seamless integrability and data online transfer via a wireless lan high speed link. These capabilities make the AOB a versatile system for a variety of applications such as ocean tomography, underwater communications, rapid environmental assessment (REA), passive and multistatic sonar and underwater target tracking. The AOB version 1 described in this report was developed between 2003 and 2004 and tested at sea during the MREA’03 and MREA’04 sea trials.
- Acoustic oceanographic buoy data report Makai Ex 2005Publication . Jesus, S. M.; Silva, A.; Zabel, F.It is now well accepted in the underwater acoustic scientific community that below, say, 1 kHz acoustic propagation models are accurate enough to be able to predict the received acoustic field up to the point of allowing precise and reliable source tracking in range and depth with only limited environmental information. This results from a large number of studies both theoretical and with real data, carried out in the last 20 years. With the event of underwater communications and the necessity to increase the signal bandwidth for allowing higher communication rates, the frequency band of interest was raised to above 10 kHz. In this frequency band the detailed knowledge of the environment - acoustic signal interplay is reduced. The purpose of the MakaiEx sea trial is to acquire data in a complete range of frequencies from 500 Hz up to 50 kHz, for a variety of applications ranging from high-frequency tomography, coherent SISO and MIMO applications, vector - sensor, active and passive sonar, etc...The MakaiEx sea trial, that took place off Kauai I. from 15 September - 2 October, involved a large number of teams both from government and international laboratories, universities and private companies, from various countries. Each team focused on its specific set of objectives in relation with its equipment or scientific interest. The team from the University of Algarve (UALg) focused on the data acquired by their receiving Acoustic Oceanographic Buoy - version 2 (AOB2) during six deployments in the period 15 - 27 September. This report describes the AOB2 data set as well as all the related environmental and geometrical data relative to the AOB2 deployments. The material described herein represents a valuable data set for supporting the research objectives of projects NUACE1, namely to fulfill NUACE’s task 3 and 4 and RADAR2, namely its tasks 2 and 3 devoted to the developement and testing of a field of sonobuoys.
- Acoustic oceanographic buoy telemetry system: an ‘advanced’ sonobuoy that meets acoustic rapid environmental assessment requirementsPublication . Silva, A.; Zabel, F.; Martins, C.The Acoustic Oceanographic Buoy (AOB) Telemetry System has been designed to meet acoustic rapid environmental assessment requirements. It uses a standard institute of Electrical and Electronics Engineers 802.11 wireless local area network (WLAN) to integrate the air radio network (RaN) and a hydrophone array and acoustic source to integrate the underwater acoustic network (AcN). It offers advantages including local data storage, dedicated signal processing, and global positioning system (GPS) timing and localization. The AOB can also be integrated with other similar systems, due to its WLAN transceivers, to form a flexible network and perform on-line high speed data transmissions. The AOB is a reusable system that requires less maintenance and can also work as a salt-water plug-and-play system at sea as it is designed to operate in free drifting mode. The AOB is also suitable for performing distributed digital signal processing tasks due to its digital signal processor facility.
- Acoustic Oceanographic Buoy Test during the MREA’03 Sea TrialPublication . Jesus, S. M.; Silva, A.; Soares, C.Environmental inversion of acoustic signals for bottom and water column properties is being proposed in the literature as an interesting concept for complementing direct hydrographic and oceanographic measurements for Rapid Environmental Assessment (REA). The acoustic contribution to REA can be cast as the result of the inversion of ocean acoustic properties to be assimilated into ocean circulation models specifically tailored and calibrated to the scale of the area under observation. Traditional ocean tomography systems and methods for their requirements of long and well populated receiving arrays and precise knowledge of the source/receiver geometries are not well adapted to operational Acoustic REA (AREA). An innovative concept that responds to the operational requirements of AREA is being proposed under a Saclantcen JRP jointly submitted by the the Universit´e Libre de Bruxelles (ULB), SiPLAB/CINTAL at University of Algarve, the Instituto Hidrogr´afico (IH) and the Royal Netherlands Naval College (RNLNC) and approved by Saclantcen in 2003 under the 2004 SPOW. That concept includes the development of water column and geo-acoustic inversion methods being able to retrieve environmental true properties from signals received on a drifting network of Acoustic-Oceanographic Buoys (AOB). A prototype of an AOB and a preliminary version of the inversion code, was tested at sea during the Maritime Rapid Environment Assessment’2003 sea trial (MREA’03) and is described in this report together with the results obtained.
- Acoustic Oceanographic Buoy testing during the Maritime Rapid Environmental Assessment 2003 sea trialPublication . Soares, C.; Jesus, S. M.; Silva, A.; Coelho, E.This paper proposes an innovative concept that responds to the requirements of acoustic REA as the integration between a network of sophisticated Acoustic-Oceanographic Buoys (AOB) and online ocean properties inversion algorithms. A prototype of the system, in- cluding one sonobuoy and a preliminary version of the inversion code, was tested at sea during the MREA'03 sea trial. The AOB is a light acoustic receiving device that in- corporates last generation technology for acquiring, storing and processing acoustic and non-acoustic signals received in various channels along a vertical line array. During the MREA'03 the AOB was deployed on a free drifting con guration. Source/receiver geom- etry was estimated from the buoy's GPS. Online processing was made possible by wireless transfer of the data and inversion was done in a range-dependent environment. Temper- ature pro les inverted from acoustic signals in two frequency bands on near
- Acústica submarina em alta frequência. Teoria, simulação e resultados experimentaisPublication . Jesus, S. M.; Silva, A.; Rodríguez, O. C.Um dos maiores obstáculos à utilização da acústica submarina para exploração e observação do oceano (tomografia, geo-acústica, etc) são as dimensões, peso, consumo de energia e custo dos equipamentos normalmente utilizados para transmissão e recepção de sinais acústicos. A maior parte dos métodos desenvolvidos e testados nos últimos 20/30 anos foram dedicados à banda de frequências baixas, arbitrariamente colocada abaixo dos 2 kHz. No outro extremo, frequências acima dos 50 kHz foram utilizadas no mesmo período de tempo, para actividades de observação oceânica a curta distância como por exemplo sonar lateral e multifeixe, perfiladores de corrente acústicos por efeito Doppler (ADCP - Acoustic Doppler Current Profilers), com grande sucesso científico e comercial. Em 2004 foi lançada, por um grupo informal de investigadores de vários países, a “High-Frequency Initiative” (HFi) que tem como objectivo o estudo teórico, em simulação através de modelos de propagação numéricos e experimentalmente, a utilização de acústica submarina a alta frequência para observaçâo oceánica, comunicações submarinas, deteção de alvos e vigilância. HFi tem como objectivo a banda de frequência compreendida entre 2 e 50 kHz. Esta comunicação apresenta os principais problemas e desafios encontrados na HFi, assim como alguns resultados experimentais e de modelação conseguidos até à data.
- Adaptive spatial combining for passive time-reversed communicationsPublication . Gomes, João; Jesus, S. M.; Silva, A.Passive time reversal has aroused considerable interest in underwater communications as a computationally inexpensive means of mitigating the intersymbol interference introduced by the channel using a receiver array. In this paper the basic technique is extended by adaptively weighting sensor contributions to partially compensate for degraded focusing due to mismatch between the assumed and actual medium impulse responses. Two algorithms are proposed, one of which restores constructive interference between sensors, and the other one minimizes the output residual as in widely used equalization schemes. These are compared with plain time reversal and variants that employ postequalization and channel tracking. They are shown to improve the residual error and temporal stability of basic time reversal with very little added complexity. Results are presented for data collected in a passive time-reversal experiment that was conducted during the MREA’04 sea trial. In that experiment a single acoustic projector generated a 2/4-PSK phase-shift keyed stream at 200/400 baud, modulated at 3.6 kHz, and received at a range of about 2 km on a sparse vertical array with eight hydrophones. The data were found to exhibit significant Doppler scaling, and a resampling-based preprocessing method is also proposed here to compensate for that scaling.
- An environmental equalizer for underwater acoustic communications Tested at Hydralab IIIPublication . Silva, A.; Zabel, F.; Mertins, C.; Ijaz, S.; Jesus, S. M.It is known that small changes in source and receiver locations can cause significant changes in underwater acoustic channel impulse responses. At HYDRALAB III an underwater acoustic experiment was conducted to show that a source depth-shift causes a frequency-shift in the channel impulse response and that such behavior can be used to implement an environmental-based equalizer for underwater communications that compensates for the performance loss due to the source depth-shift.
- Analog 16-hydrophone vertical line array for the acoustic - oceanographic buoy - AOBPublication . Zabel, F.; Martins, C.; Silva, A.This report contains the user guide and the system reference for the analog 16-hydrophone AOB vertical line array. The array is a 67.5m long cable with 16 4m-spaced hydrophones and 17 digital thermistor sensors, distributed along its length. Each module has a dedicated preamplifier to increase signal strength at the hydrophone output and drive a balanced line to transmit the signal to the surface buoy's electronics. The thermistor sensors are digital devices that transmit the temperatures values through a 1-wire digital serial bus. This report presents the system user guide and the system reference guide. The user guide has instructions for system deployment, everyday usage and maintenance. The system reference guide is intended for specialized technicians for system repairing and/or upgrade.